<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>点击发送</button>
    <button>点击取消</button>
    <script>
        const btns = document.querySelectorAll('button')
        let x = null
        // 标识变量
        let isSending = false
        btns[0].onclick = function(){
            // 判断标识变量
            if(isSending) x.abort()//如果正在发送，则取消该请求，创建一个新的请求
            x = new XMLHttpRequest()
            isSending = true
            x.open('GET','http://127.0.0.1:8000/delay')
            x.send()
            x.onreadystateChange = function(){
                if(x.readyState === 4){
                    // 修改标识变量
                    isSending = false
                }
            }
        }
        btns[1].onclick = function(){
            x.abort()
        }
    </script>
</body>
</html>